OK... this is what I found. I think the "edge break" of the zelos mount violates the Picatinny spec. Reference :
https://www.biggerhammer.net/picatinny/1913_specs.pdf It appears that the Zelos radius of the apex of the outer angular sections of the mount is quite a bit "fatter/softer" than my Picatinny rails on my AR uppers. So that edge is not as sharp and the edge break is too "soft / rounded" (if that makes sense). The slot depth does not extend far enough down to align with the vertex points on the edge of the Umarex mount. So what is going on is that the scope mount is being pulled down onto the mount and the recoil lugs are experiencing interference with the bottom of the slots on the zelos mount. In other words, I need the slots on the zelos rail mount a little bit deeper.
View attachment 524201 Below is a picture of my zelos. See how the edge of the rail is rounded bit? If you note, the slot itself is raised up just a little above the edge of the vertex rail. BTW, the zelos rail is likely fully "cast" and not machined. Take a close look, and you can see that there is sort of a light-gray line under the slot which kinda hints at the roundness. That slot is also just a tiny bit raised above the vertex point of the outer angle. It should bottom out closer to the center of the vertex, and the vertex should be sharp. Run your finger on this edge, and it feels very soft.
View attachment 524202 Here's a rail on one of my AR uppers in the image below. The rail's vertex edge on my machined AR uppers are significantly more "sharp", even though the images that I captured don't represent that well. The bottom of the slot is closer, although still slightly offset, to being coincident with the vertex.
View attachment 524208 That bit of thicker edge break of the cast rail of the Zelos causes the interference fit with the the Nikon scope base recoil lug screws. If you have a two piece clamp for your picatinny accessories, one that has the tightening screws well above the slots, you are likely good to go. If you have a mount that uses recoil lug screws that depend on tight tolerances at the bottom of those rail slots, your mount may not work. Or if you have a recoil lug screw mount that is designed for out of spec rails tolerances (which would be the smart thing to do if you were designing accessories), you are likewise good to go.
I've got to put about a half inch riser on this gun anyway in order to get the ergonomics to work better, as the Nikon mount that I have is too low. The Zelos itself has very little or no drop to the stock, and the cheek piece is too high such that I have to cant my head to look through the scope. I'm hoping that if I get a picatinny interposer to lift it up, that the interposer will have loose enough tolerances to fit bot rifle and scope mount. Otherwise I will need to pull those rail(s) and touch up those slots a little on my mill.
Bottom line is that the scope mount depends on the rail meeting the mil std spec. The Nikon mount is unforgiving in tolerance, and this is likely why this mount is no longer in production. Coupling that unforgiving Nikon mount to the sloppy tolerance of the Umarex rail, and it is a no-go.
Ugh. I just want to shoot this thing already
